Optical fibers capable of transmitting optical signals containing a large amount of information over long distances and with relatively low losses have become important components of modern communication networks. The development of such communication networks has led to a concurrent need, in various applications, for selectively controlling specific wavelengths of light within the optical fibers.
Bragg filters commonly also referred to as Bragg gratings are well known and commonly employed filters for filtering particular light wavelengths in various applications. By way of example, they are commonly used for compensating chromatic dispersion in optical fibers, for stabilizing the frequency of semi-conductor lasers, for wavelength division multiplexing (WDM) and for stabilizing and flattening the gain of optical fibers. Bragg gratings are also commonly used in instrumentation applications such as in sensors for the measurement of strain, temperature and hydrostatic pressure. They are further commonly used as narrow band wavelength-selective reflectors for fiber lasers.
The conventional Bragg grating typically includes an optical fiber in which the index of refraction along the core thereof undergoes periodic perturbations or modulations. These perturbations may be equally spaced in the case of an unchirped grating or may be unequally spaced in the case of a chirped grating.
Bragg gratings in optical fibers are conventionally fabricated by providing a fiber having a core doped with one or more materials sensitive to ultraviolet light, such as a fiber having a core doped with germanium oxide. The fiber having a doped core is exposed at periodic intervals to high intensity ultraviolet light emanating from a laser or other suitable source.
The ultraviolet light interacts with the photosensitive dopants to produce the perturbations in the index of refraction. The appropriate period spacing of the perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, a pair of interfering beams or other suitable means.
In use, light of a proper wavelength is reflected when it encounters the refractive index modulation whereas the remaining wavelength passes essentially unimpeded. The Bragg grating hence behaves as a wavelength-selective reflector having a characteristic spectral response.
In a uniform grating, the strongest reflection of light occurs at a so-called Bragg wavelength λB. The Bragg wavelength λB is typically equal to twice the effective grating period. In other words, λB=2nΛ where Λ is the spatial period of the index modulation in the fiber grating and n is the average effective refractive index of the guided mode at the position of the grating.
Accordingly, any environmental condition affecting either n or Λ will also affect the Bragg wavelength. As it turns out, both the distance or spatial period Λ between successive perturbations of the index modulation and the refractive index n of the grating are temperature dependent. Indeed, the spatial period Λ of the index modulation increases with temperature as a result of thermal expansion of the fiber. Also, so-called thermo-optic effect induces an increase in the refractive index n of fibers submitted to an increase in temperature.
These two effects combine to produce an overall increase of the Bragg wavelength with temperature. This temperature dependent increase of the Bragg wavelength is however typically primarily imputable to change in refractive index n as a result of the thermo-optic effect.
The spatial period Λ of the index modulation or distance between successive perturbations is also typically increased when the optical fiber is stretched under the action of a tensile load. The tensile load induced increase in the spatial period Λ of the index of modulation, in turn, again leads to an increase of the Bragg wavelength. The increase in the spatial period Λ of the index of modulation caused by a tensile load imparted on the fiber is typically partially offset by an ensuing reduction in the refractive index n through a so-called photo-elastic or stress-optic effect.
The Bragg or resonance wavelength of an unpackaged fiber Bragg grating shifts nearly linearly with temperature variations and load-induced stress. The dependence of Bragg wavelenghts to temperature and load-induced stresses may be used advantageously in situations wherein Bragg gratings are incorporated in hydrostatic pressure, temperature, strain or other suitable types of sensors.
The dependence of Bragg gratings to load induced stresses and often more importantly to temperature is however disadvantageous when the Bragg grating is used in applications such as communication systems often requiring a good stability of the spectral response thereof. Indeed, as optical channel space becomes narrower for higher capacity communication systems, the requirements have become increasingly stringent for controlling and stabilizing the center wavelength of Bragg gratings.
In many optical communication systems, such as those employing wavelength division multiplexing, it is important that the carrier wavelength of each channel is maintained at a substantially precise value. Typically, acceptable variations in the value of the carrier wavelength of each channel are in the range of about ±0.1 nm.
Furthermore, for most optical communication systems, it is essential that the grating wavelength remains constant over the expected temperature range. Since commercial communication systems and advanced communication networks typically operate over an extensive range of temperature, the thermal dependence of Bragg gratings greatly limits their widespread use. Accordingly, for the accurate and reliable long term operation of devices such as gain flattening filters (GFFs), dense wavelength division multiplexing (DWDM) systems, dispersion compensators using chirped Bragg gratings or the like, suitable temperature compensation means are required.
The prior art is replete with both intrinsic and extrinsic methods and devices used for constructing various types of packages adapted to support Bragg gratings in such a way as to render their wavelength insensitive to temperature changes. Intrinsic methods and devices make use of the fiber properties themselves for athermally supporting Bragg gratings.
The intrinsic methods are typically used for sensing physical parameters other then temperature such as strain, hydrostatic pressure or the like. Intrinsic methods typically rely on a pair of gratings. A first grating is used to measure the chosen physical parameter while also reacting to a change in temperature. The second grating is used in parallel for calibration purposes. The second grating measures the change in temperature only hence allowing for correction a posteriori of the physical parameter as measured by the first grating. Intrinsic methods and devices have proven to be unsuitable for telecommunication applications wherein the spectral response of each individual Bragg gratings must be stabilized against relatively small temperature fluctuations.
Extrinsic devices and systems require an extra material in order to compensate for the thermal sensitivity of Bragg gratings. Extrinsic methods and devices may be classified according to whether they are of the active or passive type. With the active type, certain parameters are continuously monitored and dynamically controlled with a feedback loop. The Bragg wavelength may be corrected, for example, by controlling the temperature of the fiber using Peltier elements or by controlling the strain in the fiber using a piezoelectric elements. Although active thermal stabilization is relatively effective, it nevertheless suffers from severe drawbacks including costly implementation, potentially costly power consumption and inherent complexity potentially leading to reliability problems.
Passive temperature compensation devices make use of the inverse and generally linear relationship between the change in strain and the change in the value of the Bragg wavelength in Bragg gratings. Such devices also typically rely on the fact that the length and modulation period of a given Bragg grating are determined by the distance between two anchoring points used for mounting the Bragg grating under tension to the device. In general terms, they typically operate by controlling the elongation with temperature of the optical fiber containing the Bragg grating.
The structure of passive temperature compensation devices is typically designed so that the distance between the anchoring points and, hence, the modulation period of the grating decreases as the temperature increases. On the other hand, the index of refraction of the fiber increases with the temperature. The increase in the index of refraction is mainly imputable to the thermo-optic effect. It also results from a reduction in the stress-optic effect. The reduction in the stress-optic effect is, in turn, imputable to the decrease in tension as the fiber expands thermally and as the anchoring points move closer to one another.
The modulation period and refractive index thus display variations of opposite signs as the temperature fluctuates. Athermalization is achieved when these variations cancel out and the Bragg wavelength remains constant as the temperature fluctuates.
Mathematically, the variation with temperature of the Bragg wavelength is described by the following equation:             Δ      ⁢                           ⁢              λ        B                    λ      B        =            [                        α          α                +        ξ        -                              p            e                    ⁡                      (                                          α                α                            -                              α                f                                      )                              ]        ⁢    Δ    ⁢                   ⁢    T  
Where αα is the coefficient of thermal expansion characterizing the thermal behavior of the distance between the anchoring points. The first term on the right hand side of the equation represents the effect of the change in length of the fiber. The second term represents the influence of the thermo-optic effect. The third term represents the change in the stress-optic effect in the fiber.
Athermalization is achieved when                               α          α                =                                                            p                e                            ⁢                              α                f                                      +            ξ                                1            +                          p              e                                                          (        2        )            
As long as the fiber remains under tension, the thermal expansion of the fiber is irrelevant except through its impact on the stress optic effect since the length of the grating is determined by the distance between the anchoring points. The initial strain in the fiber must hence be sufficient to keep it under tension over the full span of the operational temperature range.
Also, the level of applied tension is irrelevant in establishing the temperature dependence of the Bragg wavelength other than insuring that the fiber length is indeed controlled by the package. As a result, slight adjustments to the initial tension applied to the fiber can be used to fine-tune the absolute position of the characteristic spectral response of the grating.
If the optical parameters involved were to be constant, athermalization could be achieved at all temperatures. However, the optical parameters vary slightly with temperature and, hence, athermalization is only achieved at a given temperature around which the Bragg wavelength displays a generally parabolic variation with temperature.
Passive temperature compensation devices may be classified according to whether they use a material having an intrinsic negative coefficient of thermal expansion or at least two by-materials together providing a so-called differential expansion effect. When materials having an intrinsic negative coefficient of thermal expansion are used, the support material by itself tends to stabilize the Bragg wavelength around its initial value.
Although, theoretically interesting, structure using materials having an intrinsic negative coefficient of thermal expansion suffer from numerous drawbacks including that suitable materials are relatively scarce or difficult to produce and, hence, relatively expensive Also, the coefficient of thermal expansion of such materials needs to be precisely matched to the properties of the optical fiber, hence requiring a precise control of the material formulation.
Furthermore, the coefficient of thermal expansion of such materials needs to be relatively constant from one sample of material to another that may prove to be difficult to achieve in practice. In other words, it is particularly difficult to provide a negative coefficient of thermal expansion material that precisely compensates for temperature variations without any overcompensation or undercompensation.
With differential expansion temperature compensation devices, the fiber containing the Bragg grating is attached to a structure made of at least two materials having different and typically positive coefficients of thermal expansion. The multi-material structure is configured and sized so that the different rates of expansion between the structural components supporting the fiber induce a negative elongation or contraction of the fiber with increasing temperature. The fiber is pre-stretched at low temperature and allowed to relax as the temperature increases.
The distance between the anchoring points at which the fiber grating is fixed is given by the following equation:       L    a    =            ∑              i        =        1            N        ⁢                  c        i            ⁢              L        i            where c=+1 or −1 depending on the geometry of the structure and Li is the length of the ith element of the structure.
Accordingly, the coefficient of thermal expansion of the structure is given by the following equation:       α    α    =                    1                  L          α                    ⁢                        ⅆ                      L            α                                    ⅆ          T                      =                            ∑                      i            =            1                    N                ⁢                              c            i                    ⁢                      L            i                    ⁢                      α            i                                                ∑                      i            =            1                    N                ⁢                              c            i                    ⁢                      L            i                              
Where αi is the coefficient of thermal expansion of the ith material used. For example, when the structure is made up of only two distinct materials, the above equation is reduced to:Lα=L1−L2                 and       α    α    =                              α          1                ⁢                  L          1                    -                        α          2                ⁢                  L          2                                    L        1            -              L        2                    
The variation of the Bragg wavelength in such a structure is described by the following equation:             Δ      ⁢                           ⁢              λ        B                    λ      B        =            [                                                                  α                1                            ⁢                              L                1                                      -                                          α                2                            ⁢                              L                2                                                                        L              1                        -                          L              2                                      +        ξ        -                              p            e                    ⁡                      (                                                                                                      α                      1                                        ⁢                                          L                      1                                                        -                                                            α                      2                                        ⁢                                          L                      2                                                                                                            L                    1                                    -                                      L                    2                                                              -                              α                f                                      )                          +        η            ]        ⁢    Δ    ⁢                   ⁢    T  
Where η represents the effect of the behavior of the adhesive used for anchoring the fiber to the structure at the anchoring points.
Various types of temperature compensation devices using differential expansion are known. For example, U.S. Pat. No. 5,042,898 issued Aug. 27, 1991 and naming William W. Morey et al. as inventors teaches a temperature compensated optical wave guide device wherein a portion of an optical fiber containing a Bragg grating is secured at each side thereof to a different one of two compensating members. The compensating members are made of materials with such coefficients of thermal expansion relative to one another and to that of the fiber material as to apply longitudinal strains to the fiber, the magnitude of the longitudinal strains varying with temperature so as to compensate the changes in the Bragg wavelength attributable to changes in temperature. Numerous other documents teach variations or additional features based on the basic structure disclosed in U.S. Pat. No. 5,042,898.
One of the major drawbacks associated with conventional differential expansion-type temperature compensation structures relates to the fact that such structures are deprived of a suitable and reliable means for adjusting the value of the Bragg wavelength. The need for providing a suitable Bragg wavelength adjustment means has been recognized in the past. For example, U.S. Patent Application Publication U.S. 2002/0141700 A1 published Oct. 3, 2002 and naming Richard L. Lachance et al. as inventors discloses a device including a hollow structure having a threaded and a free member projecting therein respectively from opposed ends. An optical fiber is mounted in tension inside the hollow structure through longitudinal fiber-receiving bores in both members. The optical fiber has an anchor point affixed to each member. A grating is positioned between the anchoring points. The hollow structure and the members have a coefficient of thermal expansion selected so that they together compensate for the temperature dependency of the Bragg wavelength.
One of the main objects of the invention disclosed in U.S. Patent Application Publication U.S. 2002/0141700 A1 is to provide an athermal packaging where the Bragg wavelength is easily adjustable. Adjustment of the Bragg wavelength is accomplished by varying the tension on the fiber. The tension on the fiber is varied by modifying the relative positional relationship between the threaded member and the hollow structure. This positional relationship is, in turn, modified by rotating the free and threaded members together relative to the hollow structure. A nut may be provided to allow fine-turning of the resonant wavelength.
Although the structure disclosed in the hereinabove-mentioned patent application publication provides some improvement over other prior art devices by incorporating relatively fine-tunable resonant wavelength adjustment means, it nevertheless suffers from other drawbacks. One of these drawbacks is the inherent interdependency between the adjustment of the tension imparted on the fiber and a corollary adjustment of the spacing between the anchor points.
Indeed, because of its structural characteristics and because of the inherent method of manufacturing the package disclosed in the herein-above mentioned patent application, any modification of the tension imparted therewith on the fiber necessarily implies that the spacing between the anchor points will also be modified in a predetermined direction. Hence, when the tension is increased, the distance between the anchor points is also necessarily increased. Conversely, when the tension imparted on the fiber is decreased, the distance between the anchor points id necessarily decreased.
Since the modification of the tension imparted on the fiber and the distance of the anchor points is inherently interdependent, the adjustment of the center or Bragg wavelength and of the rate of wavelength drift per temperature change or wavelength excursion by temperature change is also inherently interdependent. The inability to allow for independent adjustment of the Bragg wavelength and of the wavelength excursion by temperature change may prove to be unsatisfactory in numerous situations. For example, some components such as certain types of GFFs only allow for a relatively small margin of error for both the tuning of the Bragg wavelength and athermicity.
Also, with the structure disclosed in U.S. Patent Application Publication U.S. 2002/0141700 A1, for a given package size, the only means of accurately varying the distance between the anchor points is through rotation of the threaded member. Consequently, in situations such as when the coefficient of thermal expansion of the materials varies from one structure to another or when the structure needs to be manufactured using materials exhibiting coefficients of thermal expansion different from that for which the device has been designed or used, the displacement range of the thread may prove insufficient to allow for adequate tuning or, alternatively, the threaded portion of the threaded member may need to be oversized in order to allow for adequate tuning. This, in turn, may lead to potentially inacceptable increases in the overall size of the structure.
The structure disclosed in U.S. Patent Application Publication No. 2002/0141700 A1 also suffers from failing to provide a means for ensuring precise, ergonomic and reliable positioning of the attachment components used for fixing the fiber to the members about the anchoring points during assembly. Since the performance of the structure is highly dependent on the repeatability of the manufacturing process, this may prove to be a major drawback. As a result, more stringent manufacturing is required thus leading to reduced yields and limited performance. In addition, the design of the above-mentioned device does not afford a post-manufacturing adjustment to accurately tune the response of the grating to precisely and repeatably achieve the desired temperature sensitivity specifications.
The need for allowing adjustment of the spacing between the anchoring points of the fiber to the supporting structure has been recognized in the past. For example, U.S. Pat. No. 6,377,727 issued Apr. 23, 2002 and naming Stavros Dariotis et al as inventors discloses a temperature compensating package for a fiber Bragg grating device in which the fiber Bragg grating is written to the fiber prior to the temperature compensation being set.
The device includes a housing member having a longitudinal channel defined therein by first and second side walls. First and second thermal compensation members are sized in dimension to fit within the longitudinal channel and are fixed within the latter on opposite sides of a longitudinal coupling region.
The first and second thermal compensation members each have a top surface including a first region proximal to the coupling region and a second region distal to the coupling region. The first and second thermal compensation members have a second coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the housing member.
Although the device disclosed in U.S. Pat. No. 6,377,727 allows for customization of the spacing between the fiber anchoring points, it fails to teach and, in fact, teaches away from providing a means for adjusting the spacing between the anchoring points and, hence, the tension within the fiber once the fiber is anchored to the structure. Hence, this structure also fails to provide means for allowing independent adjustment of both the central wavelength and the wavelength excursion per temperature differential. The structure also suffers from failing to provide a means for allowing accurate, ergonomic and reliable attachment of the fiber to the structure about the fiber anchoring points. Accordingly, there exists a need for an improved temperature compensating optical component packaging structure.